Signals used in mobile communication devices, such as a cell phone, base station apparatuses for them, network infrastructure devices, such as a server and a router, large-sized computers, and the like are being increased in the speed and capacity. In accordance with this, printed wiring boards mounted on these electronic devices are required to adapt to high-frequency signals, and a substrate material having excellent high frequency properties (low dielectric constant and low dissipation factor) that enable reduction of a transmission loss is demanded. As examples of the applications handling high-frequency signals, there can be mentioned the above electronic devices, the ITS field (in connection with automobile and traffic system), and the indoor short-distance communication field. It is expected that the printed wiring boards to be mounted on these devices will be further required to use a substrate material exhibiting a low transmission loss.
Further, in view of the environmental problems encountered in recent years, mounting of electronic parts using a lead-free solder and achieving flame retardancy free of a halogen are demanded, and therefore the material for a printed wiring board is needed to have higher heat resistance and more excellent flame retardancy than conventional.
Conventionally, as a resin composition used in a printed wiring board required to have a low transmission loss, one using a polyphenylene ether (PPO, PPE) and a thermosetting resin in combination has been proposed. Specifically, examples of such resin compositions include a resin composition containing a polyphenylene ether and an epoxy resin (see, for example, PTL 1), a resin composition using a polyphenylene ether in connection with a cyanate ester resin having a low dielectric constant among the thermosetting resins (see, for example, PTL 2), a resin composition using a polyphenylene ether in connection with a bismaleimide compound having high heat resistance among the thermosetting resins (see, for example, PTL 3), a resin composition using a polyphenylene ether in connection with a crosslinking polymer, such as a polybutadiene resin (see, for example, PTL 4), and a resin composition using a polyphenylene ether in connection with a crosslinking monomer, such as triallyl isocyanurate (see, for example, PTL 5).
However, the resin compositions described in the above-mentioned PTL's 1 to 5 are unsatisfactory collectively in the high frequency properties in the GHz region, the adhesion to a conductor, the thermal expansion property, and the flame retardancy. In addition, the compatibility of polyphenylene ether with a thermosetting resin is low, and therefore the resultant resin composition is unsatisfactory in heat resistance. When the polyphenylene ether ratio in the resin composition is increased for suppressing the deterioration of the high frequency properties, the resistance to chemicals (solvents), heat resistance, and formability of the resin composition tend to be insufficient.
Further, a method in which a polyphenylene ether and various types of phenols are reacted to lower the molecular weight simultaneously with introducing a functional group, such as an amino group, into the polyphenylene ether (for example, PTL 6) has been known. In the resin composition using a polyphenylene ether and a thermosetting resin in combination obtained by this method, like the above-mentioned resin compositions, the compatibility and formability are improved, but the heat resistance, glass transition temperature, and high frequency properties tend to be poor, and further the flame retardancy is likely to be unsatisfactory.
Meanwhile, the present inventors have proposed a resin composition having a polyphenylene ether resin and a polybutadiene resin as a base, wherein the resin composition can be improved in the compatibility, heat resistance, thermal expansion property, adhesion to a conductor, and the like by performing semi-IPN formation in the production stage (A-stage) of a varnish (resin composition) (for example, PTL 7). However, the substrate material recently used for a printed wiring board is not only required to adapt to high-frequency signals, but also required to have higher adhesion to a conductor, low thermal expansion property, high glass transition temperature, high flame retardancy, and the like due to the demands for an increase of the density, high reliability, and adaptability to consideration of the environment.
For example, for securing the microwiring forming properties and high reflow heat resistance while maintaining the high frequency properties (suppressing the increase of a transmission loss caused due to roughness of a conductor or the like), the adhesion to a conductor is desired to be 0.6 kN/m or more, in terms of a copper foil peel strength as measured using a low profile copper foil (Rz: 1 to 2 μm) having very small surface roughness on the side to be bonded to the resin.
Further, the substrate material for printed wiring board used in the application of network related devices, such as a server and a router, is needed to be stacked into the increased number of layers as the density of the wiring board is increased, and therefore the substrate material is required to have high reflow heat resistance and through-hole reliability. The glass transition temperature of the material as a yardstick for the above properties is desirably 190° C. or higher, further desirably 200° C. or higher, and the low thermal expansion property (in the Z-direction at the Tg or lower) is desirably 45 ppm/° C. or less, further desirably 40 ppm/° C. or less. For achieving low thermal expansion property, the incorporation of an inorganic filler into the resin composition is effective; however, in a multilayer printed wiring board with the increased number of layers, for surely obtaining the flow properties of the resin for circuit packing, the amount of the inorganic filler used is restricted. Therefore, it is desired that even when the amount of the inorganic filler incorporated is as relatively small as 25% by volume or less, based on the volume of the resin composition, the resultant resin composition secures the above-mentioned required values.
With respect to the high frequency properties, when using a general E glass substrate, the substrate material desirably has a dielectric constant of 3.7 or less, further desirably 3.6 or less, and desirably has a dissipation factor of 0.007 or less, further desirably 0.006 or less. Furthermore, the substrate material is increasingly strongly needed to satisfy the above-mentioned required values in a 10 GHz or more band which is a frequency band higher than the conventional 1 to 5 GHz band for the dielectric properties values.